This invention relates to a voltage-to-current converter comprising an input for receiving an input signal, a first transistor of a first conductivity type having a collector coupled to an output for supplying an output signal, an emitter coupled to a first impedance, and a base coupled to the input via the emitter-base junction of a second transistor of a second conductivity type. The second transistor has its collector coupled to a first power-supply terminal and the base of said first transistor also is coupled to a second power-supply terminal via the collector-emitter path of a third transistor of the second conductivity type. The base of the third transistor is coupled both to the second power-supply terminal via the emitter-base junction of a diode-connected fourth transistor and to a second impedance via the collector-emitter path of a fifth transistor of the first conductivity type. The invention also relates to a balanced voltage-to-current converter comprising a pair of identical voltage-to-current converters.
Such a voltage-to-current converter is known from U.S. Pat. No. 4,330,744 and in principle operates in the same way as the well-known emitter-follower arrangement, the current in the collector instead of the voltage on the emitter of the first transistor being taken as the output signal.
An emitter-follower arrangement has the drawback that the output current increases as a non-linear function of the input voltage because the base-emitter voltage of the first transistor varies as the input voltage increases.
The influence of the base-emitter voltage of the first transistor in the known voltage-to-current converter is compensated for by coupling the base of the first transistor to the input via the emitter-base junction of the second transistor of opposite conductivity type. The base-emitter voltages of the first transistor and the second transistor are equalised indirectly by means of a compensation circuit comprising the third and the fourth transistor, which are of the same conductivity type and which are arranged as a current mirror, and the fifth transistor, which is identical to the first transistor and which has its emitter coupled to an impedance equal to that to which the emitter of the first transistor is coupled. The current mirror keeps the collector current of the fifth transistor equal to the emitter current of the second transistor so that the base-emitter voltages of the second and the fifth transistor, and hence those of the second and the first transistor are approximately equal to each other. This known compensation circuit only operates with the desired accuracy if the current mirror ratio is unity and if the second and the fifth transistor have equal saturation currents. Since the second and the fifth transistors are of opposite conductivity types, equal saturation currents are difficult to realize in practice.